Water erosion is a matter of concern for the sustainability of agricultural systems in the world, and especially in the Mediterranean basin. The limitations of the traditional erosion measurement methods have prompted the use of environmental tracers to provide additional information for implementing conservation strategies at different scales. There are different approaches to the tracer selection with respect to their properties, methods of application and their detection.
Given the difficulties found in the use of tracers to fulfil all the requirements for a reliable assessment of the erosion and sediment dynamics defined by Zhang et al. (2001), new tracers are appearing every year. The main objective of this dissertation is the development of a simple and cheap tracer for its use in field and laboratory water erosion experiments. In a preliminary stage, two sets of tracers were considered; rare earth oxides (La2O3, Pr6O11, Nd2O3 and Sm2O3) and iron oxides (magnetite Fe3O4, hematite -Fe2O3 and goethite FeOOH). In a subsequent stage, the rare earth oxides were discarded due to the complexity and uncertainties involved in their analytical determination, in addition to their high acquisition and analysis costs.
Chapter 1 reviews the existing tracer methods adopted in erosion experiments, distinguishing between them radionuclides, rare earths, fingerprinting, and magnetic oxides. The methods are also described and compared and their advantages and disadvantages are discussed.
Chapter 2 describes the laboratory tests designed and rainfall simulation experiments to understand the behaviour and potential of magnetite as a sediment tracer in four Mediterranean soils of a varied textural class.
The next two Chapters are devoted to the exploration of iron oxides as erosion tracers. They were used at hillslope scale under simulated and natural rainfall to understand soil movement by water erosion.
Chapter 3, analyses the erosion dynamics in a cotton crop plot planted in a furrowbed system under rainfall simulations at a small scale using magnetite, and at a hillslope scale by the combination of magnetite, hematite and goethite during a sprinkler irrigation test. An erosion model was also used to understand and extrapolate the dynamics of water erosion in agricultural systems under different scenarios.
In Chapter 4 the sediment tracking procedure using magnetic iron oxide was set up in olive orchard plots combining laboratory and field magnetic susceptibility measurements. The variation in magnetite content was used to estimate the contribution of each area within the plots to total soil losses caused by natural rainfall events and the soil redistribution after tillage.
Finally, Chapter 5 is a general discussion on the work including the most relevant conclusions in it. Although the use of magnetic iron oxides implies some limitations such as, the difficulty of obtaining a uniform tagging of soil profile, it is possible to quantify the contribution of the different sources or deposition areas of sediment under different soil managements and crops. Therefore, the tracers based on iron oxides constitute a useful tool, which is complementary to traditional soil loss measurements of erosion processes.